strange noise - 98 dodge caravan

i have a 1998 dodge caravan se 3.3 v6 that just recently began making what can only be described as a "vroom" noise when accelerating. my brother-in-law said it sounded like an indy car.

this only happens when giving it gas, and when the interior climate control fan is turned on. it does not matter whether the ac is turned on or not, but there is no sound (other than normal engine purr) when the fan is turned off. my guess is it's the fan.

i have two questions:

1.) what could be causing this if not the fan?

2.) how difficult is it to replace?

thanks!

edit: i should note that none of the dummy lights are on, all fluids are fine, and all gauges are fine.